Metabolic syndrome covers many cardiovascular risk factors including hypertension, dyslipidaemia, obesity, type 2 diabetes, pancreatic β-cell dysfunction, and atherosclerosis. A diet varying in fat or carbohydrate contents contributes to energy metabolism of animals including humans. Long chain fatty acids are major source of energy and important components of the lipids that comprise the cellular membranes. They are derived from food and synthesized de novo from acetyl-CoA through complex sets of reactions. Cholesterol is also derived from food and synthesized from acetyl-CoA through equally complex reactions. The conversion of carbohydrates into acylglycerides through de novo fatty acid and cholesterol synthesis involves at least 12 and 23 enzymatic reactions, respectively. Expression levels of the genes encoding these enzymes are controlled by three transcription factors, designated sterol regulatory element-binding proteins (SREBPs), SREBP-1a, -1c and SREBP-2 (Brown and Goldstein, 1997; Osborne, 2000). These membrane-bound proteins are members of a class of the basic helix-loop-helix leucin zipper family of transcription factors (Brown and Goldstein, 1997; Osborne, 2000; Tontonoz et al., 1993). Unlike other leucin zipper members of transcription factors, SREBPs are synthesized as an ER-membrane-bound precursor, which needs to be proteolytically released by two proteases bound to the Golgi membrane, Site-1 and Site-2 proteases, in order to activate transcription of target genes in the nucleus (Brown and Goldstein, 1997; Sakai et al., 1996).
The proteolytic activation of SREBPs is tightly regulated by sterols through the interaction with SREBP cleavage-activating protein (SCAP), an ER-membrane-bound escort protein of SREBPs (Goldstein et al., 2006; Hua et al., 1996). When sterols accumulate in the ER membranes, the SCAP/SREBP complex fails to exit the ER to the Golgi, and thereby the proteolytic processing of SREBPs is suppressed. SREBPs are key lipogenic transcription factors that govern the homeostasis of fat metabolism.